This invention relates to a silicon etch solution and method and more specifically to a silicon etch solution and method which is resistivity specific.
The principles involved in wet etching silicon wafers have been known since the late 1950's. Early interest centered on chemical wafer polishing or on developing etch pits to reveal crystal imperfections. Most widely used were solutions containing HF as a reducing agent, water and acetic acid as moderating or diluting agents, and one or more oxidizers such as HNO.sub.3, H.sub.2 O.sub.2, Br.sub.2, and I.sub.2. These compositions had very high etch rates of 1 to 40 microns/min. An advantage of these etchants was the smoothness of the resulting wafer surface.
In the 1960's and 1970's, new chemical etchants for silicon were developed which added versatility and variety to silicon etching capabilities. For example, two new anistropic etchant systems, KOH/H.sub.2 O and ethylene diamine/pyrochatechol (EDP), were explored for use in the fabrication of micromechanical devices. These etches attack the &lt;100&gt; plane 30 to 400 times faster than the &lt;111&gt; plane. New oxidizers such as CrO.sub.3 were added to the HF/acetic acid/water system to reveal defects for the evaluation of epitaxial layers.
Interest in the HF/HNO.sub.3 /acetic acid/water (HNA) system was renewed to develop etches for mesa diode fabrication where resistivity specific etch rates were desired. In the early 1970's it was reported that an etching solution composed of 1 part HF, 3 parts HNO.sub.3, and 8 parts CH.sub.3 COOH by volume would etch highly doped (greater than 1.times.10 exp 17/cm.sup.3) single crystal silicon much faster than lightly doped material. The etch rate was 0.7 to 3 micron/min. for silicon having a resistivity of less than 0.01 ohm-cm. When the silicon resistivity was higher than 0.068 ohm-cm, no etching occurred.
During the manufacture of certain semiconductor devices, it is usually necessary to etch a layer or layers of polysilicon which may be doped to various levels. For example, doped polysilicon films are used in the fabrication of VLSI chips where they may serve as gate electrodes for FET structures, as polysilicon base or emitter regions in bipolar devices, or as resistors. In every case, the use of polysilicon films is predicated on the ability to etch line patterns with high resolution and sharply defined edges. Various methods of plasma etching are commonly employed to form resist-defined patterns in polysilicon films. Directional ion etching gives excellent line resolution and edge definition. Furthermore, since the polysilicon film patterns to be etched may include SiO.sub.2 films which are partly exposed to the etch, an anisotropic plasma etch will prevent undercutting in the SiO.sub.2 region. However, in fabricating bipolar devices with a polysilicon base, the polysilicon film has to be removed from the region of the future emitter. This requires a reasonably high etch rate for the doped polysilicon film and either no attack or at least a very slow etch rate of the silicon crystal underneath. No plasma or ion etch method is presently known which satisfies such an "etch stop" requirement. Additionally, ion etching introduces a certain amount of surface damage which is generally detrimental to the characteristics of semiconductor devices made in such damaged regions. An example of the surface obtained when RIE is used to etch doped polysilicon overlying an intrinsic or lightly doped surface is shown in FIG. 1. The uneveness of the surface is a result of the lattice damage which has occurred. A further problem with plasma etching is that the etchants attack the walls of the stainless steel chamber, causing nickel and iron to enter the etch atmosphere. These elements can then inbed in the wafer and result in device leakage.
With these considerations in mind, a wet chemical etch could offer significant advantages in the fabrication of certain types of semiconductor devices. HNA in a 1:3:8 volume ratio is unsuitable for this purpose because of its extremely high etch rate. Its etch rate in doped polysilicon is as high as 350 Angstroms/sec and, therefore, it does not lend itself very well to controlled etching of thin films. The result is incomplete etching if the process is interrupted too early or severe undercutting if the time limit is exceeded. Diluting the etchant with water or acetic acid slows the etch rate, but results in spotty surfaces due to polysilicon residue. A second etch solution has been used to achieve a specular surface, but this results in an additional step which is undesirable because of the additional level of process control required and because of the additional possibility of contamination of the wafers in the second etch bath.